I have just spoken to some travellers in southern Ethiopia who came via Tunisia, Algeria, Niger, Chad, Sudan & into Ethiopia. Fuel availability was no problem, and although petrol is (sometimes MUCH) mre expensive, certainly not worth changing the engine for.
It's worth taking lots spares if you have the little in line filters (or get a better filter installed).
It's also worthwhile taking a bit of (genuine) chamois leather that you can tie over the mouth of a funnel. This way, if you buy fuel from 220l drums, decant into a jerry can and then pour into the tank via the chamois lined funnel. This filters out everything, including water.